Physiological parameters in animals are measured using sensors which are placed near, on or under the skin. Wires then carry the signal from the sensor to an external amplifier and display unit. This method has a number of undesirable limitations, some of which include: the introduction of movement artefacts; restraints of movement exacerbating an unnatural environment; a potential source of infection, and; reliability problems with wires becoming tangled, breaking, or being bitten.
Some systems presently exist for the wireless monitoring of animals, however these systems have power management problems. Typically, the wireless systems that are presently available require a battery to be provided to power the sensor. This battery must be carried by the animal. The battery is often bulky which can cause difficulties when providing the sensor unit within the animal. Also, there is an inability to remotely undertake long term recordings of physiological parameters because the batteries need to be removed so that they may be replaced or recharged.
Wireless supply of power to biosensors has been attempted, but these systems either require a tightly controlled coupling between the biosensor and power source, or can only supply sufficient power for monitoring slowly changing parameters, such as temperature. Controlling the power transferred to the sensor can be difficult because the power available varies depending on location and orientation of the sensor with respect to the power source. Excess power is dissipated as heat in the sensor. This is obviously highly undesirable causing discomfort or harm to the animal in which the sensor has been implanted, and exacerbating an unnatural environment for the animal.
Contactless power supplies that transfer power inductively have been extensively developed. These have a primary conductive path (usually a cable arranged on an elongate track) which is energised by a power supply to produce an electromagnetic field about the primary path. One or more power pick-ups are provided closely adjacent to the path. Each pick-up has a tuned circuit which receives energy from the field and uses this to supply a load. These power supply systems are typically adapted to supply power over a carefully controlled relatively short air gap of approximately 1 cm.
To power a biosensor, for example in an animal in a defined space such as an enclosure or a cage, the power transfer system must deal with greater physical separation and arbitrary orientation between the primary conductive path and pick-up.
U.S. Pat. No. 6,345,203 discloses magnetic vector steering for powering multiple implant devices. It also refers to communication with an implanted device via the electromagnetic filed which energises the implanted device. The energy status of an implanted device can be monitored. However, this does not address problems with heat dissipation. Furthermore, although the implants are referred to as “arbitrarily oriented”, the system relies on a known configuration of implanted devices in relation to the primary coil. The implants are disclosed as being provided in a fixed location with respect to the primary field generating coils. Thus, the problems of variable distance and random orientation of implants in relation to the primary coils are not addressed.